The present invention relates to a discharge lamp device and especially to a discharge lamp drive device suited for backlight sources for liquid crystal display units.
Conventional liquid crystal display units used for electronic devices such as personal computers, car navigation systems use cold cathode fluorescent lamps as backlight sources for liquid crystal units. By the way, the cold cathode fluorescent lamps for this backlight source are required to have a high performance such as high brightness, low power consumption, small size, or long life according with the spread and the progress of electronic devices such as personal computers for high performance. For such demands, an outer electrode type fluorescent lamp is attracting attention. As one configuration of this outer electrode type fluorescent lamp, a lamp with a following structure is known. That is, the fluorescent lamp is composed of a glass tube in which a rare gas mainly composed of xenon (discharge medium) is enclosed airtight, on the inner surface of which a phosphor film is formed. An outer electrode, which is wound spirally around the glass tube along almost, the entire length, and an inner electrode is provided at least at one end of the glass tube.
Such outer electrode type fluorescent lamp is, generally, driven and lighted by a high frequency pulse source. The high frequency pulse source is composed of a signal drive circuit, which generates high frequency pulse signal, and an inverter circuit including a pulse transformer to which the output pulse signal of the signal drive circuit is supplied.
Now, in a liquid crystal display unit, a function is required that the luminance of the display panel is adjusted in accordance with its use circumstances. That is, in liquid crystal display unit, more effective image display is possible by properly selecting brightness etc. in accordance with an image displayed or a place where the display unit is operated. Adjusting the luminance of backlight source, that is, by a light control performs such luminance adjustment on the liquid crystal display screen. The light control of the outer electrode type fluorescent lamp described above is performed by varying the numbers of the output pulse per unit time put out of the high frequency pulse source. That is, the output pulse from the high frequency pulse source supplies a high frequency pulse of 200 pulses per second, for example, to the outer electrode type fluorescent lamp. Decreasing the number of the pulses, however, the luminance of the fluorescent lamp can be decreased. Now assuming that, the luminance of a fluorescent lamp is maximum when the high frequency pulse of 200 pulses per sec. is supplied to the outer electrode type fluorescent lamp, the light control ratio will be 50% if the number of pulses per sec. is decreased to, for example, 100 pulses per sec
However, in the light control of conventional outer electrode type fluorescent lamp, if the lighting control ratio is decreased to 1 to 5%, there was a problem that the light emission of the lamp becomes unstable, and so called a flickering might have occurred.
Taking such problems into consideration makes the present invention, and it is an object of the present invention to supply a discharge lamp device, which is capable of preventing the flickering at the low light control ratio, and to enable a stable light emission in a wide range of the light control.
The discharge lamp device according to the present invention includes a light control signal generating circuit, a drive signal circuit for generating a first, a second and a third pulse drive signals. A pulse repetition frequency of the first and the second pulse drive signals are controlled by an output signal of the light control signal generating circuit. The phases of the first and the second pulse drive signals are inverted to each other. The third pulse drive signal alternately turns on and off in accordance with the output signal of the light control signal generating circuit. The discharge lamp device according to the present invention further includes a first and a second switching devices each of which is alternately controlled open and close by said first and the second pulse drive signals supplied from the drive signal circuit, a pulse transformer having a primary coil and a secondary coil, in which a direction of a current flowing in the primary coil is switched by the first and second switching device and in which an up converted pulse voltage is generated in the secondary coil, and a flicker preventing circuit connected in parallel with the primary coil of said pulse transformer. The flicker preventing circuit is composed of a series connected circuit of a third switching device, which is made on and off in according with the third drive signal supplied from the drive signal circuit and an element having a resistance component. The discharge lamp device according to the present invention further includes an outer electrode fluorescent lamp connected with the secondary coil of the pulse transformer.
Further, in the discharge lamp device according to the present invention, the first and second switching devices are connected between the power source and the ground in series. The primary coil of said pulse transformer is connected with the connection point of said first and second switching devices and a capacitor is connected between the power source and the ground.
Further, in the discharge lamp device according to the present invention, the third drive signal supplied from the drive signal circuit turns the third switching device ON when the lighting control ratio designated by the output signal of the light control signal generating circuit is equal to or lower than a prescribed value, and turns the third switching device OFF when the lighting control ratio designated by the output signal of the light control signal generating circuit is equal to or higher than a prescribed value.
Further, in the discharge lamp device according to the present invention, the prescribed value of the light control ratio is about 20%.
Further, in the discharge lamp device according to the present invention, at least one of the first and second switching device is connected with a uni-directional element which allows an electric current to flow in one direction.
Further, in the discharge lamp device according to the present invention, the outer electrode fluorescent lamp is composed of a glass tube on the inner face of which a phosphor film is formed and a rare gas mainly composed of xenon is enclosed airtight therein, an inner electrode which is provided inside one end of the glass tube and from which a lead terminal is lead out of the glass tube, an outer electrode which is wound spirally around the outer surface of the glass tube along almost the entire length of the tube at a prescribed pitch.
Further, in the discharge lamp device according to the present invention, plurality of sets of the first and second switching devices, the pulse transformer, the flicker preventing circuit including the third switching device, and the outer electrode type fluorescent lamp are provided. The first and the second pulse-driving signal of the drive signal circuit are supplied in parallel to the first and said second switching devices of each of the plurality of sets. The third pulse-driving signal of the driving signal circuit is supplied in parallel to the third switching device of each of the plurality of sets.
The discharge lamp drive device according to the present invention includes a light control signal generating circuit, a drive signal circuit for generating a first, a second and a third pulse drive signals. A pulse repetition frequency of the first and the second pulse drive signals are controlled by an output signal of the light control signal generating circuit. The phases of the first and the second pulse drive signals are inverted to each other. The third pulse drive signal alternately turns on and off in accordance with the output signal of the light control signal generating circuit. The discharge lamp device according to the present invention further includes a first and a second switching devices each of which is alternately controlled open and close by said first and the second pulse drive signals supplied from the drive signal circuit, a pulse transformer having a primary coil and a secondary coil, in which a direction of a current flowing in the primary coil is switched by the first and second switching device and in which an up converted pulse voltage is generated in the secondary coil, and a flicker preventing circuit connected in parallel with the primary coil of said pulse transformer. The flicker preventing circuit is composed of a series connected circuit of a third switching device, which is made on and off in according with the third drive signal supplied from the drive signal circuit and an element having a resistance component.
Further, in the discharge lamp drive device of the invention, the first and second switching devices are connected between the power source and the ground in series. The primary coil of said pulse transformer is connected with the connection point of said first and second switching devices and a capacitor is connected between the power source and the ground.
Further, in the discharge lamp drive device of the invention, the third drive signal supplied from the drive signal circuit turns the third switching device ON when the lighting control ratio designated by the output signal of the light control signal generating circuit is equal to or lower than a prescribed value, and turns the third switching device OFF when the lighting control ratio designated by the output signal of the light control signal generating circuit is equal to or higher than a prescribed value.
Further, in the discharge lamp drive device of the invention, the predetermined value of the lighting control ratio is about 20%.
Further, in the discharge lamp drive device of the invention, at least one of the first and second switching device is connected with a uni-directional element which allows an electric current to flow in one direction.
Further, in the discharge lamp drive device of the invention, a plurality sets of the first and second switching devices, the pulse transformer, the flicker preventing circuit including the third switching device, and the outer electrode type fluorescent lamp are provided. The first and the second pulse-driving signal of the drive signal circuit are supplied in parallel to the first and said second switching devices of each of the plurality of sets. The third pulse-driving signal of the driving signal circuit is supplied in parallel to the third switching device of each of the plurality of sets.
Further, in the discharge lamp device or the discharge lamp drive device of the invention, the uni-directional current device is at least one selected from the group of a diode, a transistor, a MOSFET, and a photo coupler.
Further, in the discharge lamp device or the discharge lamp drive device of the invention, by inserting a device having a resistance component together with the uni-directional device such as a rectifier, which allows an electric current to flow in one direction, the stabile lighting can be more improved. The resistance component is, for example, a resistance device of from 0.05 to 10xcexa9, or an inductor device of such resistance.
With the invention described above, the flicker does not occur and a stable lighting of high luminance can be maintained and enables low power consumption in the wide range of lighting control ratio. Moreover, the light control can be performed easily with a precise control of luminance because the flicker is prevented.